1. Field of the Invention
This invention relates to an electrically conductive composition for use in a through hole of an electric component.
2. Description of the Related Art
As shown in the cross sectional view of FIG. 1A, electrical connection through through-holes of a ceramic substrate 1 has been established by applying an electrically conductive composition comprising a conductive material, such as Agxe2x80x94Pd or Agxe2x80x94Pt, to the through-hole 2, usually having a diameter of 0.3 to 0.5 mm, which are made by means of a mold or a laser beam, by screen printing, followed by calcination to form a thick-film conductor 3 on the inner wall of the through-hole 2.
With the recent tendency to size reduction and density increase in electronic equipment, the diameter of through-holes has been made as small as about 0.1 mm, and accordingly, the above-described method for electrical connection is now being replaced with a method of filling the through-hole 2 with a conductive composition as shown in FIG. 1B.
However, when a conventional conductive composition used in electronic components is put into through-holes of about 0.1 mm in diameter and calcined, the resulting conductor tends to suffer from a discontinuity as shown in FIG. 2A or separation from the inner wall as shown in FIG. 2B due to shrinkage on calcination, resulting in poor electrical reliability of the through-holes.
An object of the present invention is to prevent structural defects of electrodes, such as a discontinuity or separation of electrodes, and to provide an electrically conductive composition for use in a through hole of a electric component effective in improving electrical reliability of electrodes.
The invention provides an electrically conductive composition for use in a through hole of an electric component comprising an organic vehicle and a combination of base metal and another metal selected from the group consisting of noble metal, anti-oxidation finished base metal and Al, and the amount of the base metal to 100 parts by weight of the noble metal is about 1 to 95 parts by weight.
The above conductive composition may further comprise glass frit.
In the above electrically conductive composition, the glass frit may be lead borosilicate-based glass, zinc borosilicate-based glass or barium borosilicate-based glass.
In the above conductive composition, the member of the group may be noble metal, preferably silver or gold.
In the above conductive composition, the base metal may be copper, nickel, cobalt, tungsten or manganese. Preferably, the base metal is copper or nickel.
In the above electically conductive composition, the base metal may be copper, the member of the group may be the noble metal, and the amount of the copper to 100 parts by weight of the noble metal may be about 35 to 95 parts by weight.
In the above electically conductive composition, the base metal may be nickel, the member of the group may be the noble metal, and the amount of the nickel to 100 parts by weight of the noble metal may be about 10 to 25 parts by weight.
The electically conductive composition may further comprise platinum powder of palladium powder.
In the above electically conductive composition, the member of the group may be the noble metal, the electrically conductive composition may further comprise platinum powder, and the amount of said platinum powder to 100 parts by weight of the noble metal may be about 0.2 to 0.8 parts by weight.
The conductive composition according to the present invention provides electrodes in through holes of electric components which are free from structural defects, such as a discontinuity and separation, and secure improved electrical reliability.
Where a noble metal powder alone is used as a conductive powder forming an electrode, the electrode obtained after calcination is thinner than in its green state before calcination due to calcination shrinkage. This volume shrinkage causes a discontinuity or separation of the electrode. A base metal powder added to the noble metal powder undergoes oxidation on calcination (baking), resulting in volume expansion called oxidative expansion. That is, the oxidative expansion of the base metal powder compensates for the calcination shrinkage of the noble metal powder so that the reduction in volume after baking can be prevented. It is also possible to increase the volume after baking over that before baking. Accordingly, the present invention makes it possible to maintain the electrical connection sufficiently after calcination.
Since the base metal powder of the conductive composition should undergo oxidative expansion on calcination in formation of baked electrodes, calcination is preferably carried out in a non-reducing atmosphere, e.g., air, which is another advantage of the present invention.
The reason for limiting the amount of the base metal and the another metal is that if the amount of the base metal to 100 parts of the another metal exceeds 95 parts by weight, the resistivity of the conductive composition would become extremely large. For some uses, the loss of volume by shrinkage of a noble metal powder does not need to be filled up completely by a base metal powder. In other words, addition of a base metal powder to such an extent as to prevent occurrence of an electrode discontinuity or separation suffices for the object of the present invention.
Where, as in the present invention, an electrode comprising a noble metal powder as a primary component and a base metal powder as a secondary component is to be formed, it is preferable that the amount of the base metal to 100 parts by weight of the noble metal is about 1 to 95 parts by weight. If the proportion of the base metal powder exceeds about 95 parts by weight, the resulting conductor tends to have an extremely increased resistivity. Preferably, the base metal undergoes oxidative expansion on calcination and the amount is such that the the ratio (xe2x80x9cdimensional retentionxe2x80x9d) of the electrode thickness after calcination to the dry thickness before calcination is not lower than about 0.6, and more preferably not lower than 0.85.
Examples of the noble metal which can be used in the present invention include silver and gold. Silver is preferred from the standpoint of low resistance of the resulting conductor, ease of calcination, and the material cost. It is possible to substitute a powder of anti-oxidation finished copper, nickel, etc. for the noble metal powder. These powders, while not a noble metal, function similarly to a noble metal powder. It is also possible to use aluminum, about not being a noble metal.
Examples of the base metals which can be used in the present invention include copper, nickel, cobalt, tungsten and manganese, with copper and nickel being preferred. Copper is preferred for its ease of oxidation, its high melting point (being hardly reactive with a noble metal powder) and the material cost. Nickel is preferred for the same reasons. Both copper and nickel can be used.
When copper is used as the base metal and the nobel metal metal is used as the another metal, it is preferable that the amount of the copper to 100 parts by weight of the nobel metal is about 35 to 95 parts by weight, taking into account of a shrinkage ratio after calcination of a through-hole. When nickel is used instead of copper, the preferable amount of the nickel is about 10 to 25 parts by weight.
Copper powder, used as a base metal powder, preferably has an average particle size of not greater than about 10 micrometer. Coarse copper particles having an average particle size exceeding about 10 micrometer tend to clog through-holes, causing a conductive composition-starved state. The above-specified preferred particle size range also applies to nickel powder for the same reason.
Conventional organic vehicles can be used in the practice of this invention. Examples include acrylic resin, ethyl cellulose resin, alkyd resin, etc. dissolved in a high-melting solvent such as xcex1-terpineol and the like.
If desired, the conductive composition of the present invention may contain glass frit. The conductive composition containing glass frit is effectively applicable where electrodes and the substrate are not calcined simultaneously. Where electrodes and a substrate are formed by simultaneous calcination, the powder may undergo chemical reaction with the material of the substrate to form a chemical bond so that addition of glass frits is not always necessary. Otherwise, glass bonding is required for adhesion to the substrate. Therefore, in such cases, it is recommended to use glass frit having satisfactory adhesion strength.
Examples of useful glass frits include lead borosilicate-based glass, zinc borosilicate-based glass, bismuth borosilicate-based glass, and barium borosilicate-based glass.
The conductive composition of the present invention may further contain platinum powder or palladium powder for prevention of migration of the electrode material. In this case, platinum or palladium powder is preferably used in an amount of about 1 to 30% by weight based on the total weight of the noble metal powder, the base metal powder, glass frit (if used) and the platinum or palladium powder. If the amount exceeds about 30% by weight, the resistivity of the resulting conductor increases undesirably.
Because of the above-mentioned characteristics, the conductive composition according to the present invention is preferably used as a conductor filling through-holes or via-holes of a substrate for an electronic component. Accordingly, preferred applications of the electronic component of the present invention are multilayer ceramic substrates and hybrid IC.
According to the present invention, there is provided an electrically conductive composition which maintains electrical connection even after calcination to thereby provide baked electrodes free from structural defects, such as a discontinuity and separation, thereby securing improved electrical reliability of the electrodes.
The present invention will now be illustrated in greater detail with reference to Examples, but it should be understood that the present invention is not construed as being limited thereto.